More on the increase in size question

[petersonra]

I have some 200 HP motors at 480V, so from the table that is 240 A.

240X1.25 = 300.

The least amount of copper I can use is (2) 1/0, or I can use a single 350 KCM.

One 350 KCM conductor is about 1.65 X as much copper as two 1/0 conductors.

I have a 400A CB on the motors, so a #3 EGC.

Do I have to upsize the #3 by 1.65X if I use a single 350 instead of (2) 1/0? That would put me into a 1/0 for the EGC which would normally be adequate for an 800A CB.

Really, they need to revise this section of the code.

 

[Smart $]

...

Do I have to upsize the #3 by 1.65X if I use a single 350 instead of (2) 1/0? ...

IMO, no. My interpretation of upsizing is when you use ungrounded conductors having greater ampacity than the smallest size necessary to meet the minimum ampacity requirement. The ratio of circular mils doesn't enter the picture until the preceding is true.

 

[kwired]

Lets complicate this a little more, which I seem to have a reputation for doing:angel:

If you parallel the two 1/0 in separate raceways you will have to run two EGC also - one in each raceway.

If you parallel the two 1/0 in same raceway you may have to increase the size for number of CCC in raceway. I did not calculate but it needs looked into, if an increase is needed then a proportional increase in EGC is necessary.

If you decide to run single 350's I say you did not increase the conductror size beyond what was minimum necessary, but I see what you are getting at with it being overall a larger amount of copper, did not calculate but would imagine the single 350's will have less voltage drop than the two 1/0's.

I guess the way I see it is the choice of running parallel 1/0 vs single 350 is not any different than choosing to run aluminum Vs copper with minimum required ampacity and still using the same 3 AWG copper for an EGC.

Maybe I did not complicate it as much as I initially thought I would.

 

[don_resqcapt19]

This does bring up an issue with my idea of basing the size of the EGC on the size of the ungrounded conductor. With the example here of two 1/0s in parallel v one 350 kcmil, the EGC size using my idea would be smaller for the paralleled 1/0s than for the single 350.

This section needs a lot of work to be a usable code section. We have a bit over two years to work in it before the proposals for the 2017 code are due.

 

[taylorp]

I agree with Smart $.

My personal approach to interpreting 250.122 (B) is to:

1. Calculate the minimum conductor size needed for my circuit remembering to take account of any voltage drop, parallel installations, type of circuit, etc.,

2. Calculate the maximum rating or setting of automatic over current device in circuit ahead of equipment, conduit, etc., that a person trained in the use of the National Electrical Code would install on this circuit.

3. Choose my Equipment Grounding Conductor from Table 250.122 based on steps 1 & 2.


I also agree with BPoindexter who wrote this on the same subject in another post:

?The way that I personally apply the 250.122(B) requirement is that if the ungrounded conductors are upsized but are still less than than the Max OCPD for the EGC per 250.122 then I do not upsize. For example a #6 CU is good for up to a 200A OCPD. So if I had a 125A Circuit, requiring #1 CU, but had to upsize the ungrounded conductors to 1/0 CU for derating then I would still use a #6 CU. Otherwise you could end up with situation described above where you have #8 CU for both ungrounded and EGC conductors.?

I agree that 250.122 (B) needs work, I just don?t know what language the CMP would accept to make it simple and workable.

 

[Cow]

Parallel or single conductors sized to an OCPD do not require upsizing an EGC. Only conductors larger than called for(whether paralleled or single) for a particular OCPD require upsizing.

I wanted to say don't overthink it, but then realized that most of the code is that way. So I can understand where you're coming from....

 

[taylorp]

I agree with Smart $, BPoindexter, and Myself.

If your branch circuit needs a #6 conductor, but you only need a 50 Amp Overcurrent device, you have to remember that in 5 years some idiot will come in behind you and install a 70 Amp Overcurrent device (hopefully not more).

So the requirement in 250.122 (B) is there....................

 

[don_resqcapt19]

...

I also agree with BPoindexter who wrote this on the same subject in another post:

?The way that I personally apply the 250.122(B) requirement is that if the ungrounded conductors are upsized but are still less than than the Max OCPD for the EGC per 250.122 then I do not upsize. For example a #6 CU is good for up to a 200A OCPD. So if I had a 125A Circuit, requiring #1 CU, but had to upsize the ungrounded conductors to 1/0 CU for derating then I would still use a #6 CU. Otherwise you could end up with situation described above where you have #8 CU for both ungrounded and EGC conductors.?
...

But the code wording does not permit that method.

 

[Smart $]

This does bring up an issue with my idea of basing the size of the EGC on the size of the ungrounded conductor. ...

Hmm... wouldn't those using lower temperature-rated wire be "penalized"?

Why not base the EGC size on the ampacity of the largest ungrounded conductor, perhaps before adjustment, correction, termination considerations (i.e Table ampacity)... and upsize where the ocpd rating exceeds the conductor ampacity, such as with motor circuits... ???

 

[kwired]

Hmm... wouldn't those using lower temperature-rated wire be "penalized"?

Why not base the EGC size on the ampacity of the largest ungrounded conductor, perhaps before adjustment, correction, termination considerations (i.e Table ampacity)... and upsize where the ocpd rating exceeds the conductor ampacity, such as with motor circuits... ???

Motor circuits is where I would likely run into problems the most (outside of say a cable assembly that you can't change the size of EGC because it comes preinstalled in the cable)

I understand the reasoning for wanting a proportional increase in the EGC, my question is whether or not there has been significant study or records of actual failures in the field because the EGC was not increased, especially in the types of instances that we debate often here. Like a 40 amp circuit using 6AWG needs increased EGC, even though 6 AWG could be used with smaller EGC on a 60 amp circuit.

 

[don_resqcapt19]

Motor circuits is where I would likely run into problems the most (outside of say a cable assembly that you can't change the size of EGC because it comes preinstalled in the cable) ...

I would expect that where the motor circuit branch circuit short circuit and ground fault protection is a thermal magnetic breaker, that a cable with a standard sized EGC would be a problem most of the time. For example a motor with current from table 430.250 of 40 amps would require #8 conductors. That cable would most likely have a #10 EGC, but the code requires an EGC sized based on the branch circuit protective device that would be permitted to be rated at 100 amps for this circuit. The 100 amp device would require a #8 EGC.

 

[kwired]

I would expect that where the motor circuit branch circuit short circuit and ground fault protection is a thermal magnetic breaker, that a cable with a standard sized EGC would be a problem most of the time. For example a motor with current from table 430.250 of 40 amps would require #8 conductors. That cable would most likely have a #10 EGC, but the code requires an EGC sized based on the branch circuit protective device that would be permitted to be rated at 100 amps for this circuit. The 100 amp device would require a #8 EGC.

Exactly why I mentioned it, I do work on lots of motor installations - most below 100 HP but have connected and serviced many over the years. Majority of time I am feeding them with raceway wiring methods so there is no problem, but throw in the occasional situation where you might want to run a cable and it is easy to forget the EGC conductor in the cable may not be large enough for the higher allowed overcurrent device on a motor circuit.

 

[don_resqcapt19]

Exactly why I mentioned it, I do work on lots of motor installations - most below 100 HP but have connected and serviced many over the years. Majority of time I am feeding them with raceway wiring methods so there is no problem, but throw in the occasional situation where you might want to run a cable and it is easy to forget the EGC conductor in the cable may not be large enough for the higher allowed overcurrent device on a motor circuit.

It seems that a lot of industrial work is done using tray cable. Is this rule just ignored or do they use a non-standard cable for motor circuits?

 

[Smart $]

It seems that a lot of industrial work is done using tray cable. Is this rule just ignored or do they use a non-standard cable for motor circuits?

All [heavy] industrial projects I have been on, say over the past decade, have used for most situations standard ANSI/NEMA/ICEA tray cable, most of which all conductors were of the same size and insulation.

 

[don_resqcapt19]

All [heavy] industrial projects I have been on, say over the past decade, have used for most situations standard ANSI/NEMA/ICEA tray cable, most of which all conductors were of the same size and insulation.

I don't use much tray cable, but the last I used had an EGC that was sized to the power conductors in the cable and would not have been suitable for motor circuits. To get the cable with all of the conductors the same size, do you just order it without an EGC?

 

[kwired]

I don't use much tray cable, but the last I used had an EGC that was sized to the power conductors in the cable and would not have been suitable for motor circuits. To get the cable with all of the conductors the same size, do you just order it without an EGC?

Like any other cable they can make about anything, what a supplier decides to stock depends on what customers demand the most. Or you can bypass supplier, but likely will need to place a minimum quantity of an order.

 

[Smart $]

... To get the cable with all of the conductors the same size, do you just order it without an EGC?

I've not been involved with ordering on any of those projects, so I don't know.

What I can tell you is most (not all) were ICEA Method 1, Table E-2 color coded (http://www.buyawg.com/colorcoding.php). Some electricians I worked with called it K-2 color coding, but I have no idea where that came from. A few of the cables just had [dull] white tracers and numbers printed on each conductor's black insulation.

The only cables I recall having true EGC's were for large motor feeders, and had three bare EGC's twisted in with the ungrounded conductors and filler strands. I don't recall the conductor sizing.